1. Field of the Invention
This invention relates generally to methods and apparatus for preventing, suppressing or mitigating explosions in confined subterranean chambers, and more particularly to a method utilizing a flexible heat critical inflated bladder filled with an explosion suppressing agent or inert gas wherein the volume of the inflated bladder displaces the normal atmospheric volume of the chamber in which an explosive fuel/air mixture may otherwise accumulate, and upon failure of the bladder the explosion suppressing agent or inert gas is released to alter the ratio of the fuel/air mixture and suppress or mitigate the explosive reaction. The inflated bladder also serves as a compressible plenum or pressure accumulator to retard ultimate pressure buildup.
2. Brief Description of the Prior Art
Volatile mixtures of gas and air often accumulate in the upper portions, access opening or entryway of a confined subterranean chamber such as the manhole of sewers, service boxes, electrical vaults, mining tunnels, and other confined subterranean enclosures. These chambers sometimes house electrical distribution wiring for utilities and are usually enclosed with a cover and thus provide a protected chamber for the initial ignition of an explosion that is destructive to both human life and property. These explosions have caused multiple fatalities as well as significant property damage.
The vapors and explosive fuel/air mixtures are generally comprised of hydrocarbons in a gaseous state. The actual sources of the fuels are not always known. Further, the accumulation rate and the amount of accumulation has, to date, eluded any predictive process.
Various fuels from various sources can accumulate in confined subterranean spaces. For example, in manholes some common sources include: (1) Gas leaks in distribution gas mains that service individual customers. These leaks propagate through the soil to an area of less pressure. Manholes offer this area of lower pressure because they are exposed to ambient atmospheric conditions, in contrast to the higher pressures in the soil that are the result of the weight of the soil plus the weight of any water in the soil. (2) Accumulations of methane produced by bacterial and decaying biological material. (3) Spills of gasoline and oil from street traffic as well as other volatile wastes that are washed into manholes via normal precipitation runoff. (4) Super-heating or cooking of polymer insulation of the distribution wires that are in the manholes. This super-heating or cooking process causes the hydrocarbons in the insulation to return to the gaseous volatile components from which they were made, and is most often caused by high resistance flaws in the cable that go into thermal runaway and result in low voltage arcing.
Gaseous fuels become explosive when mixed with an appropriate amount of air. The size of the explosion (notwithstanding the effects of the stoichiometric relations of fuel-to-air) will be largely determined by the volume of the fuel and air mixture within a confined space. The larger the volume of the fuel and air mixture, the larger (and more potentially destructive) the resulting explosive event.
The optimum ratio for a fuel/air mixture to cause an explosion is approximately 9 or 10 parts air to 1 part fuel. Although this ratio varies with any particular hydrocarbon gas, the 9:1 or 10:1 mixture will usually produce an explosive event when common hydrocarbon gases such as methane, ethylene, acetylene, etc., are involved. This fuel/air ratio must be obtained and maintained if the fuel is to explode with maximum effect. If a rich mixture (too little air) or a lean mixture (too much air) is present, the resulting explosion will release less energy than would have been released if the explosion was the result of an optimum mixture. Further, if the ratio of fuel-to-air never reaches explosive potential, a rapid release of energy (explosion) will not take place. Thus, any action that will prevent or reduce the accumulation of a potentially explosive gas and air mixture will suppress, mitigate, or even possibly prevent an explosion.
Several methods have been proposed to contain or reduce the effects of explosive-like reactions resulting from fuel-air mixtures. The following patents are representative of the prior art attempts.
Geertz, U.S. Pat. No. 2,352,378 discloses the formation of a flame barrier in a mine by spraying into the mine passageway a combination of a carbon dioxide "snow" and rock dust.
Glendinning et al, U.S. Pat. No. 2,693,240 detects an incipient explosion reaction by detecting a rate of static pressure increase above a predetermined level.
Mathisen, U.S. Pat. No. 2,869,647 discloses an apparatus for detecting and suppressing explosions, in which there is a radiation detector responding to certain frequencies of radiation, and a liquid suppressant distributor having electrically ignitable explosive means for projecting the liquid.
Kopan et al, U.S. Pat. No. 3,156,908, discloses particular circuitry for detecting a flame.
Mitchell et al, U.S. Pat. No. 3,482,637 and Jamison, U.S. Pat. No. 3,515,217 disclose exploding fire suppressing material (e.g. alkali metal carbonates) as a means of suppressing a gas-air explosion.
One of the problems of the systems taught by the above listed patents is that they are susceptible to frequent reactions to false alarms because of their inability to discriminate between what is an actual condition of a possibly rapidly propagating combustion reaction of an air-fuel mixture and other disturbances that do not result in such a combustion reaction, such as a stationary flame, an electric spark, or a blast wave (resulting, for example, from an explosion deliberately initiated). Another problem with these types of systems is that of deploying a combustion suppressing material so that it provides an effective barrier to the propagating reaction. Since the actual reaction front is usually preceded by a pressure wave, there is a tendency for the suppressing agent to be blown away from the reaction front so as to diminish is suppressing action.
Richmond, U.S. Pat. No. 3,831,318 discloses an explosion detection and suppression system wherein a plurality of bags are stored in a deflated condition around the side walls of a coal mine passageway and connected with a radiation sensor, a static pressure sensor, a dynamic pressure sensor, and a data analysis computer connected with an activating means. The bags are inflated with a combustion suppressing agent upon the occurance of a predetermined rate of change in one of the sensed conditions to form a barrier in the passageway. The bags are made of a heat deteriorable material so that the heat from the combustion reaction causes the combustion suppressing agent in the bags to become exposed to the combustion reaction.
Jenkins, U.S. Pat. No. 3,990,464 discloses a heat responsive duct closing method and apparatus wherein a normally collapsed leak-proof inflatable bag is stored in the ventilating ducts of a building and automatically inflates upon activation of a smoke detector to completely seal the duct from the passage of air and smoke to block the spread of noxious smoke and fumes and prevent the access of fresh air which would contribute to the spread of a fire.
Clodfelter et al, U.S. Pat. No. 5,501,284 discloses an inflatable bag fire extinguishing system for use in ventilated or confined spaces, such as an aircraft engine compartment, wherein a porous bag is stored in a deflated condition in a confined compartment having ventilating air flowing therethrough and connected with a container containing a charge of gaseous vaporizable liquid fire extinguishing agent through an electrically operated release valve or rupture diaphragm. Upon detection of a fire, the bag is inflated with the fire extinguishing agent to block incoming ventilation air which is needed to sustain the fire, to displace a portion of residual air in the compartment, and simultaneously disperse the fire extinguishing agent into the remaining voids of the compartment through the pores of the bag. The bag may also be made with a non-porous wall portion on the upstream side.
The Jenkins, Clodfelter et al, and Richmond patents all teach inflating a collapsed bag upon detection of a potential explosion or fire or upon the occurance of a such an event to block either the spread of smoke or the propagation of the explosion while simultaneously dispersing explosion suppression or fire extinguishing agents. They do not teach (1) maintaining a bag or bladder inflated at all times to reduce the volume and displace the normal atmospheric content in the chamber and thereby reduce the amount of space in which the explosive fuel/air mixture may otherwise accumulate. Although these patents teach filling a bag or bladder with explosion suppressing and fire extinguishing agents, they do not teach (2) that the bag or bladder is heat critical at a predetermined temperature and contains a predetermined volume of inert gas relative to the volume of the chamber which is sufficient to alter the ratio of the fuel/air mixture in the chamber when the bladder fails to suppress or mitigate the explosive reaction. These patents also do not teach (3) utilizing the bag or bladder as a compressible plenum or accumulator to retard ultimate pressure buildup such that the pressures caused by a limited explosion are not transferred to the chamber cover and interconnecting ducts of the subterranean enclosure.
The present invention is distinguished over the prior art in general, and these patents in particular by a method for preventing, suppressing or mitigating explosions in a confined subterranean chamber, access opening, or entryway of an underground structure such as the manhole of sewers, service boxes, and mining tunnels. The method utilizes a flexible bladder filled with an inert gas or explosion suppressing agent, wherein the bladder is maintained in the chamber in an inflated condition and the volume of the inflated bladder occupies greater than 70% of the volume of the chamber to significantly reduce the amount of space in which an explosive fuel/air mixture may otherwise accumulate. The bladder is heat critical to disintegrate at a predetermined temperature and release a volume of the explosion suppressing agent or inert gas relative to the volume of the chamber which is sufficient to alter the ratio of the fuel/air mixture in the chamber to prevent, suppress or mitigate the explosive reaction. The inflated bladder also serves as a compressible plenum or pressure accumulator to retard ultimate pressure buildup such that the pressures caused by a limited explosion are not transferred to the chamber cover and interconnecting ducts of the subterranean enclosure.